1. Field of the Invention
The present invention relates to an image scanner of a digital copying machine, a facsimile, a scanner, and the like, particularly to an image scanner having a both-sided simultaneously scanning configuration in which both sides of an original are simultaneously scanned once and a control method thereof.
2. Description of the Related Art
Conventionally, in image scanners used in the copying machine and the like, there is known an image scanner which performs so-called “flow scan”. In the flow scan, the original is conveyed onto an original base plate glass one by one by an automatic original-feeding device, and an image of the original is exposed and scanned by an exposure device fixed at a conveyance path of the original. For example, Japanese Patent Application Laid-Open No. 2001-285595 disclosed the flow scan.
Japanese Patent Application Laid-Open No. 2004-187144 disclosed a configuration in which two image-scanning units are provided to improve productivity to scan both the sides of the original at one-time conveyance. Japanese Patent Application Laid-Open No. 7-283906 discloses a data transfer technique in the case of only one image data output line in the configuration in which the two image-scanning units are provided.
When the both-sided simultaneous scan of the original is performed in the configuration in which only one image data output line is provided, it is necessary that scan image data of the backside be stored in a temporary storage memory during scanning the surface of the original.
Usually DRAM is used as the temporary storage memory. For example, it is assumed that the image data is scanned with a 600-dpi line sensor and an A/D conversion circuit converts the image data into digital data while eight bits are set to each of R, G, and B. In this case, because an image data capacity of about 210 Mbyte is required for an A3 original, the large-capacity temporary storage memory is required, which results in a problem of increasing apparatus cost.
In order to solve the problem, for example, Japanese Patent Application Laid-Open No. 11-289427 discloses a control technique in which a scanning operation speed is decreased when the temporary storage memory capacity is run short, the temporary storage memory outputs the image data to an external device, and the scanning speed is returned when the shortage of the temporary storage memory capacity is eliminated.
In the control disclosed in Japanese Patent Application Laid-Open No. 11-289427, the scanning operation is temporarily interrupted when the temporary storage memory capacity is run short, the temporary storage memory outputs the image data to the external device, and the scanning operation is resumed when the shortage of the temporary storage memory capacity is eliminated. Therefore, the reduction of the memory capacity and the cost reduction can be achieved.
However, because the control such as the interruption and resumption of the original scanning operation is required, it is necessary to stop and start up a driving source for conveying the original, which results in a problem in that an image deformation is easily caused.
Because the image data is transferred to the external device before the data scan is completed for the whole of one surface of the original, in the case where continuous handling of the image data of one surface is required on the external device side, there is a problem in that the image data of the one surface cannot continuously be handled unless the external device includes the temporary storage memory.
In view of the foregoing, the invention provides an image scanner which can continuously transfer the image data of the one surface to the external device (image forming apparatus) without performing the control such as the interruption and resumption of the original scanning operation.